How to migrate itunes library to external drive AFTER upgrading a system

So, I've done this a bit backwards (not planned) and wonder if there is a way to get it sorted now.

I HAD my itunes library and music (and apps and backups) on my MBP. That machine died, and I created a new build with an SSD (smaller drive of course) and did NOT bring all the MUSIC over.

I DO have a copy of the HOME folder, so all the itunes folders and the MUSIC folders (And I guess mobile apps, et al.) and what I HAVE done is move the itunes folder (without the music folder) to the new HOME/MUSIC folder (same user and name!) and things seem to work, I can plug in iphones and ipods and sync new podcast, etc.

But, what I want to do is somehow POINT the itunes library that is on the newly created MBP SSD drive to the external MUSIC folder to look there for its music. It is all the same music it thought was there before, nothing new added.

Is there a simple way to do that? Is that just in Advanced and point the itunes library to the external, or does one have to do the OPTION click itunes and locate the library?

Okay, now here is the next rub. I'd LIKE to be able to download things while on the road - and not connected to the external drive. Have those files of course come LOCAL to the MBP/SSD, but when I connect to the external drive, I'd like the new music/apps files to MOVE OVER to the external drive.

I think I've read that is possible, but not sure the process for it. Can that be done?

For your simplicity, it might be best to enable "organize" and "copy to iTunes" (or whatever those two checkboxes say) in the same place.

2) usually it's best to bring the external with you, but if you don't, you can save songs locally, create a playlist of all the songs you download, then either:

a. when you get back home, reconnect to the external drive, delete the songs from the database and computer, and re-download them from iTunes in the Cloud.

or

b. when you get back home, reconnect to the external drive, delete the songs from the database (but not the files from the computer), open finder and and drag/drop those specific song files back into the iTunes window to copy them over to the external drive and re-add them to the database.